It is known to fabricate containers by blow molding, wherein a substantially tubular plastic parison commonly referred to as a “preform” is disposed in the cavity of a mold and expanded into the shape thereof by the injection of a pressurized fluid into said preform. This method lends itself well to the rapid fabrication of containers of consistently high quality.
A common variant of this process is stretch blow molding, in which a stretching rod is inserted into the preform and urged against an interior surface thereof, inducing the preform to deform along its longitudinal axis. This is particularly favored in that it enables one to exercise a greater deal of control over the longitudinal deformation of the preform, thereby enabling the production of a wider range of container shapes and sizes than by simple expansion alone.
A typical blow-molding apparatus comprises a mold, in which is provided a mold cavity in the form of the container to be produced. The preform is provided in a substantially tubular form globally resembling a test tube and which is disposed at least partially within the mold, conventionally being inserted into the mold cavity through a hole disposed in an upper surface of the mold.
Prior to being inserted into the mold, the preform is generally heated above the glass transition temperature of the thermoplastic resin from which the preform is fabricated. This will soften the preform and thus increase its propensity to deform, facilitating its formation into a container.
Preferably, a small portion of the preform protrudes from the mold, permitting the attachment of an injection head to a mouth of the preform in communication with an internal cavity of the preform. This mouth portion of the preform is usually furnished with threads, rims, or other such means for interfacing with a closure device such as a cap, and remains substantially unchanged during the container forming process while the rest of the preform is expanded into a container.
To effectuate the expansion of the preform, a fluid under pressure is injected into the cavity of the preform to induce the preform to expand. While traditionally this fluid was a pressurized gas such as compressed air, it has become known to inject a non-compressible injection liquid instead. This offers a greater degree of control over the molding process, and where the injection liquid is the product to be packaged within the container combines the forming and filling steps to realize a considerable gain in process efficiency.
While many such blow molding processes employ molds as described above, it has recently become known to form containers by free-blowing. In the free-blowing process, the preform is fixed in the molding apparatus at an open end thereof, but is otherwise unrestrained by a mold as it expands under the pressure of the injection liquid. In a common variant of this process, a partial mold is provided to define a portion of the container, for instance a shallow cup-shaped plate to define the bottom of the container, while permitting the preform to expand freely over the rest of its surface.
The free-blowing molding process is advantageous in that the dimensions of the container are a function of the volume of liquid injected therein, and in that it minimizes or eliminates the cost of providing molds.
The free-blow molding process is not without its disadvantages. During the fabrication of the container the preform cools, such cooling accelerating with the expansion of the container. In particular, the injection liquid will serve as a heat sink, drawing the heat out from the preform. This limits the maximum volume of container that may be fabricated before the preform cools to below a temperature at which expansion may occur without producing defects in the finished container.
To combat this, it is necessary to compensate for the heat lost during the forming process, presently accomplished by furnishing a preform which is thicker and heavier than would be otherwise necessary so as to compensate for the loss of heat to the injection liquid. Containers so formed are disadvantaged with a corresponding increase in thickness, weight, and expense.
It is therefore an objective of the invention to provide a container fabricating-apparatus which provides means for heating a preform undergoing expansion in a free-blowing molding process. It is a further objective of the invention to provide a method by which such an apparatus is employed.